Doc: git.txt: remove backticks from link and add git-scm.com/docs

Doc: git.txt: remove backticks from link and add git-scm.com/docs

While checking the html formatted git(1) manual page, it was noted
that the link to https://git.github.io/htmldocs/git.html was formatted
as code. Remove the backticks.

While at it, add the https://git-scm.com/docs link which one reviewer
noted had linkable section headings.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>